This invention relates to a splint made of an elongated rectangular flat strip of malleable metal, such as aluminum, which is wide enough to embrace an arm, a leg or a finger, but insufficient in thickness to provide requisite rigidity to serve as a splint. The strip is initially rollable or foldable into a compact package for storage or shipment. When unrolled or unfolded to flat form, preparatory to application on an injured extremity, the strip is first manually folded or bent transversely between its ends to provide two legs connected by said transversely curved area, and the legs are then manually bent or folded longitudinally approximately midway between the longitudinal edges of the legs into U-form, said longitudinal bends or folds extending from the leg ends to the opposite sides of said transversely bent connecting area into which they merge. The manually shaped longitudinal folds or bends produce non-uniform U-shaped transverse cross sections in the legs and impart splint-serving rigidity to the legs. The non-uniform character of the manually formed longitudinal U-shaped bends in the legs produces interrupted contact surfaces which permit passage of air between the splint and skin of the extremity to which the splint has been applied. For application to arms, legs and fingers, the convex surfaces of the two legs face each other at opposite sides of the extremity; for other applications, such as neck injuries, the legs may be given longitudinal bends or folds which make the convex surfaces of the legs face away from each other or toward each other, as required by the particular injury.